The present invention relates to electronic watthour meters and more particularly to techniques and apparatus therein for configuring a meter to monitor electrical energy consumption on various types of service networks and further to such a meter which contains apparatus for improving the accuracy of the measurement of electrical energy.
It is well known that there are several different types of electrical distribution systems or service in common use today. These systems provide power to a user facility in the form of:
1. a four wire three phase wye service; PA1 2. a three wire three phase delta; PA1 3. a three wire network; PA1 4. a four wire three phase delta service; PA1 5. a three wire single phase service; and PA1 6. a two wire single phase service.
Historically the monitoring of electrical energy consumption by a load which is connected to these services has been done by various types of induction type watthour meters designed specifically to be connected to these types of services. Meters must be configured so as to be properly connected to each of the different types of services. Unfortunately, two variants of each basic meter type are required, differing primarily by their full scale current rating. The so called self-contained watthour meter in common use today has a full scale current rating of 200 amperes. The second type of commonly used meter is called a transformer-rated meter (used with external current transformers to scale down their large current loads) and has a full scale current rating of 20 amperes. In the historical development of electromechanical or induction type meters, the self-contained and transformer-rated meters wound up with slightly different watthour constants (watthours per disc revolution). Therefore, the two different types of meters cannot be provided for in just the scale factor alone of the current sensor in the meter which is used to sense the line currents. Thus it can be seen that a need exists for a watthour meter which can be configured to accommodate the various types of distribution systems or electrical services.
Electronic registers are in common use today with induction type watthour meters. Typically the induction type watthour meter contains a pulse initiator which senses rotation of the meter disc and provides pulses proportional to energy consumption to the register. These electronic registers are typically used for the measurement of kilowatt demand and/or time of use energy consumption. In order to accumulate data representative of these types of consumption, a time base is usually required. This time base is used for interval timing of typically 5, 15, 30 or 60 minutes for calculation of kilowatt demand and for keeping time and date information in time of use meters.
Whenever an electronic register is employed with an induction type meter, generally one phase voltage is supplied to the register to provide both power to operate the register and also the line frequency for the time base. If that particular phase voltage fails, the register will cease operation. However, on polyphase induction type meters there may be up to two other phase voltages supplied to the meter. If the phase voltage supplying the 60 Hz time base to the register fails, the meter disc will continue to rotate due to the other two active phases, but the electronic register will not operate normally even if powered. Thus it can be seen that a need exists to be able to provide the line frequency time base to an electronic register from a meter if any one phase voltage is available at the meter voltage input.
For induction type meters, particularly of the transformer-rated type, it has been the practice for many years to provide "pot lamps" to indicate potential or the presence of voltage at each of the meter potential inputs. Typically, these pot lamps are energized from a secondary winding on each meter potential coil, and indicate that flux is being delivered to the meter disc. These pot lamps can also be used to indicate the presence of each of the phase voltages at the input to the meter. In earlier watthour meters these pot lamps were first incandescent bulbs (one per voltage input) and more recently these meters utilize light emitting diodes. Each of these devices draw significant power, has limited life, emits light (a target for vandalism) and is difficult to see in high ambient light conditions. Meter readers typically are expected to check each of the lamps at each monthly reading and report any problem if a lamp is not operating. Thus it can be seen that a problem will never be identified unless a meter reader or some other knowledgeable person is present at the time of loss of potential of any of the inputs to the meter. In this context, it can be seen that a need exists for insuring integrity of the potential circuits in an electronic meter down to a level at least comparable to that in induction type watthour meters while overcoming most of the shortcomings of the induction type meter.
In electricity metering, electric utility companies historically have found it desirable to measure, in addition to total kilowatt-hours (real volt amperes), power factor, KVA, or reactive volt amperes. The measurement of reactive volt amperes typically has been accomplished by using a second meter in conjunction with the conventional kilowatt-hour meter. From the reactive volt amperes and the real volt amperes, quantities such as power factor and KVA can be calculated. This second meter for measurement of reactive volt amperes, is in actuality a watthour meter connected with phase shifting transformers in the voltage circuits. Voltage phase shifts of 90 degrees result in a measurement called Vars (reactive volt amperes). Voltage phase shifts of 60 degrees result in a different measurement generally called Q or Qhours. Q is in fact a reactive measurement and may have well evolved from the fact that the 60 degree phase shift could be readily accomplished by cross phasing the meter voltage connections to a polyphase circuit at the meter, thus eliminating the need of phase shifting transformers as is required for the measurement of Vars. The requirement to provide a second meter for making these reactive measurements is expensive by the mere fact that the second meter must be employed. Thus it can be seen that a need exists to provide a single meter which is capable of measuring both kilowatt-hours and reactive volt amperes without external phase shifting transformers or the need to make special connections.
For a detailed description pertaining to electricity metering and in particular for detailed information pertaining to the various types of electrical services and distribution systems and the types of meters utilized to perform kilowatt-hour, varhour and Qhour metering, reference is made to the Handbook for Electricity Metering, Eighth Edition, published by the Edison Electric Institute.
Electric utility companies have come to expect very high levels of reliability in the metering equipment they purchase from manufacturers. In electronic metering equipment in particular, it is important to be able to tell if a piece of equipment is good or has failed without the need to perform complex or time consuming test procedures or to remove the equipment from the installation. Thus it can be seen that a need exists to be able to quickly and easily verify proper operation of key elements or circuits in an electronic meter without complex or lengthy test procedures and without having to remove the equipment from service. Further a need exists to achieve this reliability and test capability with a low cost solution which does not substantially reduce the meter equipment reliability due to increased complexity of additional circuitry.
Electronic meters employ analog amplifiers, such as those used in analog to digital converters and current to voltage converters, as well as other types of circuits and components which can introduce DC offset voltages in the entire meter contributing to inaccuracy in the measurement of power. For example, a typical DC offset error voltage might typically be one millivolt, or worst case as large as 30 millivolts, in a low cost single chip integrated circuit for a complete watthour meter which is constructed from a CMOS process. There have been techniques developed for trimming out this DC offset in high performance single chip amplifiers of the aforementioned type, but they are not considered appropriate for the many amplifiers necessary to implement in an electronic meter such as that contemplated by the present invention. It is possible to build a suitable calibration means into an electronic meter which adjusts out the effects of DC offset at the time the meter is calibrated. However these offsets can drift with time, and more significantly with temperature, thus causing changes in the electronic meter accuracy. Meter accuracy versus time and temperature are both important to electrical utilities and have limits specified in national standards. Therefore it can be seen that it is desirable to provide a means to compensate for the accumulation of DC offsets in an electronic meter and which also adapts to any changes in the DC offset that might occur during the lifetime of the meter.